Electrolytic production of ozone



June 1966 J. A. DONOHUE ETAL 3,256, 64

ELECTROLYTIC PRODUCTION OF OZONE Filed Oct. 30, 1965 3 Sheets-Sheet 1NICKEL ANODE IRON CATHODE VENT VIA REFLUX COND. VENT VIA REFLUX COND.(-78 C)NOF ABSORBER AND (-78C)NOF ADSORBER AND KI BUBBLER KI BUBBLERINLET FOR HF 8 INLET FOR ANODE FLUSH GAS T CATHODE FLUSH GAS AND WATER xSTAINLESS STEEL HEAD TEFLON INSULATION I KEL'F CUP I E & I I

I ANODE i IRON CATHODE 5 t I I 5 I i L KEL-F DIAPHRAM Fig 1 3(PERFORATED) 5 I i I i I I l n i i 7 I E I i I i N KEL-F ELECTRODEINVENTORS:

SPACER John A. Dona/we BY Willie/I A. Wilson ATTORNEY June 14, 1966 J.A. DONOHUE ETAL 3,

ELECTROLYTIC PRODUCTION OF OZONE Filed 061'.- 30, 1963 2 Sheets-Sheet 2Fig. 2

PRODUCT, Gms/KW -Hr.

HF ELECTROLYTE WATER CONTENT, W\. K,

John A. Dona/me William A. Wilson Maw ATTORNEY INVENTORS:

United States Patent 3,256,164 ELECTROLYTIC PRODUCTION OF OZONE John A.Donohue, Chicago, IlL, and William A. Wilson, Griflith, Ind., assignorsto Standard Oil Company, Chicago, 11]., a corporation or Indiana FiledOct. 30, 1963, Ser. No. 320,057 3 Claims. (Cl. 204-129) This applicationis a continuation-inpart of our copending application S.N. 14 1,175,filed September 27, 1961, now abandoned. This invention relates to theelectrolytic production of mixtures of ozone (O and oxygen difluoride(0P particularly where ozone is the predominant product.

We have discovered that excellent yields of ozone can be obtained fromhydrogen fluoride-water solutions by electrolysis conducted in aparticular manner. According to our invention, ozone is produced in highyield by carefully controlling the water concentration of the liquidelectrolyte during electrolysis. 'For best results, the liquidelectrolyte consists essentially of HF and about 2-7 percent water, byweight. Maximum ozone yield is obtained when the HF electrolyte containsabout 46% water.

In our electrolytic method of generating ozone, anelectric current ispassed through a liquid electrolyte consisting of hydrogen fluoride andnot more than about weight percent of water to produce a mixture ofgases containing ozone; oxygen difluoride, hydrogen and oxygen are alsoproduced. The amount of ozone produced is substantially greater than theamount of oxygen difluoride produced when the electrolysis is carriedout at a temperature of not more than about 50 C., preferably 20 C. to+20C., with the electrolyte consisting of hydrogen fluoride and 4-6percent of water.

The electrolytic method of the invention can be carried out in anyelectrolytic cell wherein a liquid electrolyte can be positioned in thecell and an electric current passed therethrough; more commonly,electrodes are immersed in the liquid electrolyte and provisions aremade for maintaining liquid electrolyte at the desired temperature ofoperation. Electrolytic cells suitable for the carrying out of a methodof the invention include those now used for the electrolytic productionof fluorine except that the carbon anode needs to be replaced by a metalanode. Illustrative descriptions of suitable cellsare given in Chapter8, Flourine Chemistry, I. H. Simons, editor (Academic Press, 1950). Itis to be understood that the cells will be modified by anyone ofordinary skill to mate-rials of construction suitable for use with HFsolutions. It has been found that particularly suitable materials ofconstruction for the electrodes are: anode formed of nickel and cathodeformed of iron, specifically, black iron. We

have also found that the anode and cathode compartments need not beseparated.

When an electric current is passed through a liquid electrolyteconsisting of an HP solution containing not more than about 10 weightpercent of water, gases are produced. These gases include hydrogen,molecular oxygen, oxygen difluoride (0P and ozone (0 content of the HFelectrolyte is critical and must be carefully controlled. Enough watermust be present to perrnit passage of the electric current through theliquid HF positioned in the cell and to furnish the oxygen for theproduction of the O and 0P In general, at least about 1 weight percentof water is present; that is, the electrolyte consists of 99 percent ofHF and 31 percent of water. We have discovered that upon increasing thewater content of the liquid electrolyte above about 1%, the-amount ofozone produced increases markedly and also increases with relative tothe oxygen difluoride produced. Also, as the water content of the liquidelectrolyte increases in the The water ice range of 4-6 weight percent,the yield of ozone passes through a maximum point. However, as the watercontent of the liquid electrolyte approaches toward 10 ,weight percent,the predominant position of ozone decreases and eventually the oxygendifluoride becomes predominant; so that the electrolyte used in theproduction of ozone as the desired product contains not more than about10 weight percent of water; that is, the liquid electrolyte preferablyconsists of about 93-98 percent of HP and about 2-7 percent of water.The best combination of ozone content in the product gases and yield ofozone per kilowatt-hour of power consumed in the cell is'obtained whenthe liquid electrolyte consists of about 4-6 weight percent of water andthe remainder essentially hydrogen fluoride. Since water is consumedduring production of O and 0P make-up water must be added to theelectrolyte to maintain the water concentration within the criticalrange for maximum ozone producion and yield.

It is to be understood that the hydrogen fluoride as used herein mayinclude regular commercial grade acid as well as high purity hydrogenfluoride itself. When utilizing commercial grade acid, the water contentthereof will be calculated as part of the total desired water content ofthe particular liquid electrolyte present in the electrolytic cell.

The electrolyte must be in the liquid state and s'uflicient pressuremust be maintained on the cell to keep the electrolyte in the liquidstate at the particular temperature of operation. The temperature mustnot be so low that the electrolyte freezes.

The electrolytic cell is operated at any temperature which will permitthe production of ozone. In general, the cell is operated at atemperature of not more than about C. and desirably at a temperaturebelow about 50 C. When the absolute maximum of ozone production'isdesired, the cell is operated at a temperature between about 20 C. to+20 C.

The product of the electrolysis comprises a mixture of hydrogen, oxygen,oxygen difluoride and ozone. The hydrogen may be readily separated fromthe other gaseous products by condensing these three. When it is desiredto operate with a mixture of low molecular oxygen content, the oxygenmay be removed by low temperature distillation from the ozone and theoxygen difluoride. The ozone and the oxygen difluoride may be separatedby low temperature distillation. It has been discovered that silica geladsorbs ozone in preference to oxygen difluoride and essentially pureozone may be recovered from the silica gel adsorbent. Without taking anyspecial pains to eliminate oxygen difluoride present in the spacesbetween silica gel particles, it has been possible to obtain desorbedozone containing on the order of 4 mole percent of oxygen difluoride. Bytaking special precautions, it is entirely possible to produce ozonecontaining only trace amounts of oxygen difluoride ornone at all.

ILLUSTRATIONS The cell used in this work is shown schematically in,FIGURE 1. It is composed of a Kel-F cup covered by a stainless steelcap. The use of Kel-F allows the contents of the cell to be observedduring the electrolysis. The cup was approximately 2" in diameter and 4high. The diaphragm made from $5 Kel-F sheet, was solid above the liquidlevel to prevent mixing of anode and cathode gaseous products, butperforated below the liquid level to allow free flow of the electrolyte.The nickel anode x 1%" x 2%") and the black iron cathode (%2" x 1'78 x3") were held in place by slots in the cup wall and a Kel-F spacer. Theelectrode separation was inch. Nichrome wire leads were welded to eachelectrode and insulated from the steel cap with Teflon. The steel capwas provided with two openings on each side of the diaphragm. The anodeinlet was connected, via a calibrated Kel-F transfer buret, to an HPcylinder. The fittings were of Monel, and the gas (nitrogen or helium)used to transfer the HF was also used as anode flush gas. A copper tubeconducted the flush gas to the cathode inlet. This could be easilyremoved and water or other materials inserted. The vents on both sidesof the cell were connected separately, via copper tubing, to a refluxcondenser filled with Dry Ice-acetone (78 C.). Any HF which passed thecondenser was absorbed in sodium fluoride packed tubes. The HF freegases could then be passed into glass bubblers for analysis.

Current was obtained from a DC. power line and In this reported seriesof tests the cell was equipped with a new nickel anode, and theconcentration of water was kept above about 3%. As a further precautionthe tests at lower water concentration were run at lower voltages and,therefore, lower current densities. This was done to prevent formationof even small amounts of fluorine which is one possible cause ofcorrosion at the anode.

In the first half of this series (Tests 1-5) the results were quite goodeven at low current densities. At higher current densities results wereslightly better. Then the cell was allowed to stand at room temperaturefor two weeks with the anode in contact with the HF-water elecsteppeddown with a variable resistance. The voltage trolyte. When the cell wasstarted up again the ozone and amperage were measured with standardmeters. 15 yield had dropped. A second test showed an ozone yield TABLEAdded Testing Product Test No. Water, Volts b Amps. Opera weight on,percent a Minutes u 0 013; 0 0F; 0

2. 9 6. 9 1. 2 60 18. 0 9. 8. 8 5. 8 3. 9 ,6. 6 1. 2 G0 20. 7 12. 4 10.1 6. 9 4. 7 6. 4 1. 2 60 24. 1 15. 0 8.6 6. 0 4. 6 7. 0 1. 6 45 26. 8l4. 8 l0. 4 6. 7 4. 6 7. 4 2. 0 36 27. 1 14. 6 12. 9 7. 9 5. 5 8. 0 2. O36 25. O 12. 5 14. 4 8. 1 7. 0 8. 7 2. 0 37 19. 8 9. l 16. 4 8. 5 8. 2l1. 0 2. 0 36 6. 9 2. 6 17. 8 7. 3

Remainder of electrolyte consisted of commercial HF solution containing99.9% mini.

mum HF.

Voltages in tests 4, 7 and 8 are averages over the time.

The actual moles produced are the numbers listed multiplied by 0.0001,i.e., 10- Hydrogen and oxygen also produced.

B Grams of product produced per kilowatt-hour of power consumed in theelectrolysis eel l The bubblers were filled with 200 ml. of a 2% KIsolution. After each run the free iodine was titrated by the usualmethod using thiosulfate with starch as the indicator. The fluoride ionwas also determined by the thorium chloranilate method. The amount of 0Pformed can be calculated from the fluoride analysis directly. Todetermine ozone the iodine released by 0P is first calculated andsubtracted from the analyzed iodine. The corrected value then gives theamount of ozone:

O +2KI+H 0 I +2KOH+O The following is a generalized procedure for aseries of tests. The cell was assembled and pressure tested with air at10 p.s.i.g. The diaphragm was checked by filling the cell with water andraising the liquid level on one side. The cell was then sealed and setaside overnight. If the liquid level did not equilibrate the diaphragmwas considered gas tight. The cell was then emptied and a measuredamount of water was added. It was next placed in ,an ice bath (0 C.) andconnected to transfer lines and condensers. After the desired amount ofHF was transferred to the cell, a slow flow of helium was begun on oneside of the diaphragm and a slow flow of nitrogen was begun on the otherside of the diaphragm. This gave a fairly constant pressure on thesystem, activating the KI bubblers while keeping the electrolyte levelundisturbed.

For the first minutes after electrolysis was begun the products wereusually vented. Then when the amperage and voltage had settled down andwere fairly constant, the run was started by switching the product gasesinto Kl bubblers. (Average values of voltage are reported in tests wherethey were not constant.) The test was continued until sufficientreaction occurred in the K1 bubblers. Then electrolysis was stopped andthe KI solutions were submitted for analysis. The water concentration ofthe cell was then increased and the series continued.

increase with time of operation. (These results are not reported here.)Increasing the water concentration in later Test 6 gave nearly completerecovery of ozone yield. Further increases in water concentration inTests 78 caused ozone yield to drop off. The results of these Tests 1-8are set out in the table. The critical effect of electrolyte watercontent on ozone and oxygen difluoride yields,based upon the powerconsumed in the cell, is shown in FIGURE 2.

. Several runs were made with a platinum anode. It was found that atabout 3% water the platinum corroded rapidly. As soon as the current wasturned on the solution blackened from corrosion products. In all thetests reported here with the nickel electrode, the solution remainedclear and colorless. Any corrosion of nickel was small, or at least itadhered tightly to the anode.

In other tests, it has been observed that the presence of a diaphragm toseparate anode and cathode compartments is not essential to successfuloperation of the cell.

Ozone itself has a considerable use in the purification of drinkingwater, the treatment of industrial wastes and the deodorization of airand sewage gases. The compositions containing ozone and oxygendifluoride are useful where oxidizing power is desired and there is noproblem of effect on animal life. For example, the mixed composition ofthe invention may be used in the treatment of industrial wastes whichare not disposed of directly in sources of drinking water. Also, themixed compositions may be used in chemical reactions where the presenceof combined fluorine is not detrimental to the quality of the finalproduct; for example, in ozonolysis reactions with olefins.

Thus having described the invention, what is claimed is:

1. A method of electrolytically generating ozone which method comprisespassing an electrical current at a voltage in the range of about 6.0 v.to about 11.0 v. through a liquid electrolyte positioned in anelectrolytic cell said liquid electrolyte being maintained at atemperature in the range of about 20 C. to about +20 C., said liquidelectrolyte consisting of hydrogen fluoride and about 2-7 weight percentof Water, and withdrawing fro said cell gases containing ozone.

2. The method of claim 1 wherein said electrolyte composition is: HF,93-98%; and water, 27%.

3. A method of electrolytically generating ozone and oxygen difluoride,with the ozone being in the greater amount, which method comprisespassing an electrical current at a voltage in the range of about 6.0 v.to about 11.0 v. between the electrodes of an electrolytic cell, saidcell containing a liquid electrolyte into which said electrodes areimmersed said liquid electrolyte being maintained at a temperature inthe range'of about ,20 C. to about +20 C., said liquid electrolyteconsisting of hydrogen fluoride and about 4-6 weight percent of Water,and withdrawing from said cell gases containing ozone and oxygendifiuoride, the amount of ozone being substantially greater than theamount of oxygen diflouride.

References Cited by the Examiner UNITED STATES PATENTS 2,034,458 3/1936Calcott et a1 204128 X OTHER REFERENCES Chemical Abstracts I, volume 37,column 61972, 1943.

Chemical Abstracts II, volume 42, column 4471 f, 1948.

Chemical Abstracts III, volume 54, column 13900e, 1960.

JOHN H. MACK, Primary Examiner.

5 H. M. FLOURNOY, Assistant Examiner.

1. A METHOD OF ELECTROLYTICALLY GENERATING OZONE WHICH METHOD COMPRISESPASSING AN ELECTRICAL CURRENT AT A VOLTAGE IN THE RANGE OF ABOUT 6.0 V.TO ABOUT 11.0 V. THROUGH A LIQUID ELECTROLYTE POSITIONED IN ANELECTROLYTIC CELL SAID LIQUID ELECTROLYTE BEING MAINTAINED AT ATEMPERATURE IN THE RANGE OF ABOUT -20*C. TO ABOUT +20*C., SAID LIQUIDELECTROLYTE CONSISTING OF HYDROGEN FLUORIDE AND ABOUT 2-7 WEIGHT PERCENTOF WATER, AND WITHDRAWING FROM SAID CELL GASES CONTAINING OZONE.